Modern radiation therapy techniques include the use of Intensity Modulated Radiotherapy (“IMRT”), typically by means of an external radiation treatment system, such as a linear accelerator, equipped with a multi-leaf collimator (“MLC”). Use of multi-leaf collimators in general, and an IMRT field in particular, allows the radiologist to treat a patient from a given direction of incidence to the target while varying the shape and dose of the radiation beam, thereby providing greatly enhanced ability to deliver radiation to a target within a treatment volume while avoiding excess irradiation of nearby healthy tissue. However, the greater freedom IMRT and other complex radiotherapy techniques, such as volumetric modulated arc therapy (VMAT, where the system gantry moves while radiation is delivered) and three-dimensional conformal radiotherapy (“3D conformal” or “3DCRT”), afford to radiologists has made the task of developing treatment plans more difficult. As used herein, the term radiotherapy should be broadly construed and is intended to include various techniques used to irradiate a patient, including use of photons (such as high energy x-rays and gamma rays), particles (such as electron and proton beams), and radiosurgical techniques. While modern linear accelerators use MLCs, other methods of providing conformal radiation to a target volume are known and are within the scope of the present invention.
Several techniques have been developed to create radiation treatment plans for IMRT or conformal radiation therapy. Generally, these techniques are directed to solving the “inverse” problem of determining the optimal combination of angles, radiation doses and MLC leaf movements to deliver the desired total radiation dose to the target, or possibly multiple targets, while minimizing irradiation of healthy tissue. This inverse problem is even more complex for developing arc therapy plans where the gantry is in motion while irradiating the target volume. Heretofore, radiation oncologists or other medical professionals, such as medical physicists and dosimetrists, have used one of the available techniques to develop and optimize a radiation treatment plan.
One of the common criteria for radiation treatment planning may be that a target volume attains the target coverage prescribed thereto. For example, a target coverage may be expressed by a statement that “at least 98% of the target volume should be covered by the prescribed dose level of 40 Gy.” In practice, a target coverage may be enforced by a separate plan normalization step after an optimization has been performed based on other dosimetric criteria, where the dose level is scaled by adjusting the number of monitor units (MU) associated with the optimized control point sequence.
In cases where a tumor has metastasized, there may be multiple treatment targets within a treatment area of a patient. In concurrent treatment of multiple targets, the plan normalization solution may be sub-optimal, since a treatment plan may have different target coverages for different targets so that a single scaling factor may not be able to correct the target coverages for all targets.
Therefore, it is desirable to have optimization techniques that can attain uniform target coverages for multiple targets simultaneously in radiation treatment planning.